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Homework Help: How many moles.

  1. Feb 16, 2013 #1
    1. If I have this chemical formula : Mg(NO3)2 and I have one mole of it, how many moles of each element do I have ?

    Mg: 0,16mol
    N2: 0,19 mol
    O6: 0,65 mol

    1. The problem statement, all variables and given/known data

    Would my answer be any good ?
  2. jcsd
  3. Feb 16, 2013 #2


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    It's hard to tell. You have typed in some numbers without showing any work.
  4. Feb 17, 2013 #3
    Sorry, here it is : Formula n=m/M n=number of moles m=mass in g M=Molar mass


    Mg= 24g
    O=16.6= 96

    Total= 24+28+96=148g/mol

    Mg :24/148=0,16 mol
    N2: 28/148=0.19 mol
    O6: 96/148=0.65 mol

    Would that be enough ? Thanks !
  5. Feb 17, 2013 #4
    If you have one mole of the compound, and if there is one atom of magnesium per compound, then how could you have anything other than one mole of magnesium?
  6. Feb 17, 2013 #5
    You can't use that formula for this particular question. The formula is the actual mass of the molecule divided by the molecular mass of the molecule, which is the mass of 1 mole of a molecule.

    For this question, you would simply multiply the number of moles of the molecule by the number of each respective element in the molecule. Since you have 1 Mg, you multiply 1 mole * 1 = 1 mole of Mg in the molecule.
  7. Feb 17, 2013 #6


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    Hi chemistry1!
    In one mole of that compound there are 2 moles of N atoms.
  8. Feb 18, 2013 #7
    Thanks everyone, I understood the logic behind it. :P I was just not viewing it in the right way.
  9. Feb 18, 2013 #8
    If you had only two magnesium nitrate molecules how many atoms of each element would you have? You can just draw this situation on a piece of paper and count up the atoms. For instance you would have something like this on your paper (for two Mg(NO3)2): Mg, Mg, N, N, N, N, O, O, O, O, O, O. Now just do the same for a mole...just kidding but you should be able to see the relationship of number of moles of a given molecule and the number of moles of the constituents of the molecule. It's important to remember that for problems such as these we aren't concerned with structure or anything fancy. You can just as well write MgN2O6 or the long way that I have above and you would be able to work out the problem. The way it is written in the question will give much more information which isn't necessary for this particular problem but which is useful for a Chemist IE we know the anions are nitrates, a well known molecule important in a lot of chemical processes.

    A useful exercise may be to do analogous thought experiments with everyday objects you are familiar with. For instance how many tires do 4 cars have? How many steering wheels? how many rearview mirrors? How did you figure that out? How about a mol of cars?
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